Oil well pipe and method for making the same

Abstract: An oil well pipe comprises an oil well pipe body and a sliding layer formed on the outside of said oil well pipe body. A method for making a oil well pipe comprises the steps of preparing an oil well pipe body and forming a sliding layer on the outside of said oil well pipe body. The sliding layer is made from a material having a stiffness S(t) of 10 ⁻⁵ to 10³ N/m² after one year or more has elapsed at a working temperatureof the oil well pipe, said stiffness S(t) being expressed by the following equatuion using a deformation amount d(t)after t seconds, a frictional force τ and a thickness h of the sliding layer: S(t) = 3h · τ/ d(t).

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a oil well pipe for casing, which is not destroyed by ground subsidence. 2. Description of the Related Arts An oil well for extracting crude oil is formed by a tubing, which is an oil well pipe for drawing up crude oil from the oil reservoir to the ground, and a casing, which is an oil well pipe for protecting the tubing. Since most oil wells are bored substantially in the vertical direction, a downward pulling frictional force acts on the oil well pipe for casing (hereinafter called oil well pipe) due to ground subsidence caused by the extraction of crude oil etc. In the conventional oil well, however, the aforementioned frictional force acting on the oil well pipe is not always high, so that the possibility that the oil well pipe is destroyed by the frictional force is not always high. Therefore, the measures for reducing the frictional force have not been taken. US-A-4 483 396 discloses a landfill gas well which compensates for ground shift by providing a tubular gas extracting means comprising a pipe and an upper pipe, which is sufficiently smaller than the pipe, in order to enable the pipe and a closely fitting pipe sleeve to be freely received within the bore of the pipe. That is, D1 relates only to mechanical devices being not deformable and not showing a viscous behavior such as sleeves and O-rings. GB-A-2 107 422 discloses the treatment of pipes to increase the ductile fracture resistance in the axial direction by wrapping a plurality of continuos unidirectional high-strength fibers around a pipe in the form of a wrapping. US-A-4 036 539 relates to a drill pipe string wherein the non-bearing parts of the string and the stabilizer are coated with an adherent plastic coating which is characterized by a low coefficient of friction, abrasive resistance, low surface tension, durability, toughness and slickness, in order to prevent the sticking or balling of the drill string within the bore or casing due to build-up of drill cuttings and the like about the stabilizer blades. "Oil and Gas Journal", vol. 76 (1978), no. 36, pp. 107-111, describes a method that permits the bending of polyurethane-coated pipes by coating a pipe with a polyurethane foam and applying a melt extruded polyethylene sheet on the coated pipe. US-A-4 796 670 relates to a drill pipe protector for reducing the frictional forces between the drill string and the casing, wherein the protector includes at least an outer portion and surface made of an elastomeric material, by polishing the interior surface of the casing and allowing a thin lubricating film to form between the drill pipe protector and the casing to maintain the reduced rotary torque. US-A-4 818 148 relates to a pile foundation as a structural foundation for reducing the frost heaving force acting on the pile in a frigid area by providing a pile member, a smooth-surfaced plastic covering member and adhesion means. Specifically the surface of the covering member has a low coefficient of friction so that the frost heaving force is reduced by making the active layer of the soil slide on the covering member. The steel pipe of this document is suitable for an oil well pipe for casing. Recently, the problem of oil well pipes destroyed by ground subsidence has become serious as the oil wells become deep. The oil well pipes buried in a great depth are subjected to a high underground pressure, so that the acting frictional force is high, and the destruction of oil well pipes cannot be neglected. For this reason, the measures for reducing the frictional force acting between the ground and the oil well pipe have been taken, and the development of an oil well pipe which is not destroyed by ground subsidence has been desired. SUMMARY OF THE INVENTION It is an object of the present invention provide an oil well pipe which is less prone to be destroyed by ground subsidence by reducing the frictional force acting on the oil well pipe and a method for making the same . To achieve the object, the present invention provides an oil well pipe for casing according to claim 1 and a method according to claim 5, the preamble of both claims corresponding to US-A-4 818 148. BRIEF DESCRIPTION OF THE DRAWINGS FIG 1. shows a sectional view of an oil well pipe of the present invention; FIG 2. shows a schematic view illustrating a method for making an oil well pipe of the present invention; and FIG 3. shows a schematic view illustrating viscous behavior of a deformable layer. DESCRIPTION OF THE PREFERRED EMBODIMENT To achieve the object, the present invention provides an oil well pipe for casing comprising an oil well pipe body; and a deformable layer formed on the outside of said oil well pipe body, wherein said deformable layer is composed of a material having a viscous behavior at the working temperature of the oil well pipe, and said deformable layer is deformed by a shearing stress when a frictional force is applied to the oil well pipe from the outside. If a frictional force due to ground subsidence acts on an oil well pipe buried vertically under the ground, the deformable layer formed on the outside of the oil well pipe and having viscosity is pulled down by a frictional force and is deformed, thereby reducing the frictional force acting on the oil well pipe body. Specifically, as shown in the schematic view of FIG 3., when a constant shearing stress (frictional force) t acts on the upper surface of a deformable layer 21 showing viscous behavior and having a thickness h, sliding deformation (viscous behavior) as indicated by the broken line in the figure occurs after t seconds. The relationship between the deformation amount d(t) after t seconds and the frictional force τ can be expressed by Equation (1) using the stiffness S(t) and the thickness h of the sliding layer. τ = S(t) · d(t)/3h It can be seen from Eq.(1) that the use of a material having a low stiffness S(t) for the sliding layer decreases the frictional force τ . The inventors of the present invention conducted model experiments and found that the frictional force τ should be 2 x 10 3 N/m 2 or lower in order for the oil well pipe to be less prone to be destroyed by ground subsidence. Further, we found that the destruction of oil well pipe can be prevented almost completely by reducing the frictional force τ to 2 x 10 2 N/m 2 or lower. From these results, we determined a value of 10 3 N/m 2 as the stiffness S(t) of the sliding layer which does not cause destruction of oil well pipe assuming that the thickness h of the sliding layer is 2 mm, the deformation amount d(t) of the sliding layer after one year is 20 mm, and a frictional force of 2 x 10 3 N/m 2 continues to act. It was found that if the stiffness S(t) after one year has elapsed is 10 3 N/m 2 or lower, the oil well pipe is less prone to be destroyed, and if the stiffness S(t) is 10 2 N/m 2 or lower, which is the case corresponding to a frictional force of 2 x 10 2 N/m 2 or lower, the destruction of oil well pipe does not occur substantially. In other words, a material having a stiffness S(t) after one year exceeding 10 3 N/m 2 cannot reduce the frictional force because the deformability is insufficient, having a high possibility of destruction of oil well pipe. It was also found that if the stiffness S(t) after one year is lower than 10 -5 N/m 2 , the sliding layer is softened and melted. The stiffness S(t) of the material in accordance with the present invention decreases gradually before one year has elapses. However, after one year has elapsed, the stiffness S(t) scarcely change, being substantially equal to the value at the time when one year has elapsed. Therefore, the range of the stiffness S(t) required is the same after one year has elapsed. As described above, the oil well pipe in accordance with the present invention has a reduced frictional force between the subsiding ground and the oil well pipe body, preventing the destruction of the oil well pipe, so that a material for the sliding layer is selected in accordance with the temperature at which the oil well pipe is used. That is to say, the material used for the sliding layer must naturally show the intended viscous behavior in the intended temperature range. Therefore, when the oil well pipe is used in the temperature range of 100 to 130°C, which now poses a problem of destruction of oil well pipe, a material having a stiffness S(t) in the range of 10 -5 to 10 3 N/m 2 in the temperature range of 100 to 130°C, preferably a material in the range of 10 -5 to 102 N/m 2 , should be selected as the material for the sliding layer to prevent the destruction of oil well pipe. Needless to say, it is preferable that this condition should be met even in the temperature range other than the above range.' Preferably, a material which has a stiffness S(t) in the above range in a temperature range as wide as possible is selected. At present, the temperature which oil well pipes are subjected to is 130°C or lower. As the result of various investigations on materials meeting the above conditions, it was found that several materials can be used as the sliding layer. First, asphalt can provide a material having a stiffness S(t) after one year in the range of 10 -5 to 10 3 N/m 2 in the temperature range of 40 to 100 °C by mixing straight asphalt and heated asphalt in the predetermined proportions. The asphalt thus obtained can contain additives such as resin, fat and oil, rubber, pigment, and filler. Polyethylene can also be used. The softening point of polyethylene can be changed by changing its density. When the oil well pipe is used in the temperature range of 100 to 130 °C, polyethylene having a density of 0.930 g/cm 3 or lower must be selected to meet the condition of the above range of stiffness S(t) in this temperature range. Polyethylene having a density exceeding 0.930 g/cm 3 has a stiffness S(t) after one year of 10 3 N/m 2 or higher at 100°C, so that it does not soften sufficiently, being incapable of performing the function as the sliding layer. Contrarily, polyethylene having a density of 0.930 g/cm 3 or lower has a softening temperature lower than 100°C so that it can perform the function as the sliding layer because its stiffness S(t) after one year at 100°C is 10 3 N/m 2 or lower. For polyethylene having a low density (0.930 g/cm 3 or lower), the lower limit of density need not be specified because the condition that the stiffness S(t) after one year at 130°C is 10 -5 N/m 2 or higher is met. Maleic acid modified polyethylene can also be used. The softening point of maleic acid modified polyethylene can be changed by changing its density. When the oil well pipe is used in the temperature range of 100 to 130°C maleic acid modified polyethylene having a density of 0.930 g/cm 3 or lower must be selected to meet the condition of the above range of stiffness S(t) in this temperature range. Maleic acid modified polyethylene having a density exceeding 0.930 g/cm 3 has a stiffness S(t) after one year of 10 3 N/m 2 or higher at 100°C so that it does not soften sufficiently, being incapable of performing the function as the sliding layer. Contrarily, maleic acid modified polyethylene having a density of 0.930 g/cm 3 or lower has a softening temperature lower than 100 °C so that it can perform the function as the sliding layer because its stiffness S(t) after one year at 100°C is 10 3 N/m 2 or lower. For maleic acid modified polyethylene having a low density (0.930 g/cm 3 or lower), the lower limit of density need not be specified because the condition that the stiffness S(t) after one year at 130 °C is 10 -5 N/m 2 or higher is met. Maleic acid modified polyethylene has high adhesion properties to the steel of the oil well pipe body because it has a polar group of maleic acid. Ethylene-vinyl acetate copolymer can provide a material for meeting the condition of the above range of stiffness S(t) in the intended temperature range by selecting the optimum composition. Because vinyl acetate has a lower softening temperature than ethylene component, the stiffness S(t) can freely be selected in a fixed range by changing the compounding proportion. To meet the condition of the above range of stiffness S(t) in the temperature range of 40 to 130°C the content of vinyl acetate in ethylene-vinyl acetate copolymer is set at 5 to 60 percent by weight. Also, maleic acid may be allowed to react with ethylene-vinyl acetate copolymer to yield maleic acid modified ethylenevinylacetate copolymer in order to form the sliding layer. In this case, the adhesion properties to the steel of the oil well pipe body are high because maleic acid is a polar group. For the same reason, a material in which maleic acid modified polyethylene is added to ethylene-vinyl acetate copolymer may be used as the sliding layer. An additive etc. may be mixed with the aforementioned resins as necessary. Such a sliding layer can be formed on the outer peripheral surface of the oil well pipe body, for example, by extrusion coating. Next, the reason for providing a protective layer will be described. The protective layer serves to prevent the damage to the sliding layer and the oil well pipe body when the oil well pipe is transported or buried. The present invention includes the case where the oil well pipe has a protective layer as necessary. For the protective layer, a material is used which does not soften in the environment in which the oil well pipe is used. When the upper limit of the working temperature is 130°C a material having a softening point exceeding 130°C may be used. Such materials include polypropylene and polyolefin resin such as ethylene-propylene block copolymer. Preferably, polypropylene or ethylene-propylene block copolymer having a softening temperature of 140 to 170°C should be used. When the sliding layer formed from maleic acid modified polyethylene is coated with ethylene-propylene block copolymer, both materials are desirable ones because they have high adhesion properties. Like the sliding layer, an additive etc. can be mixed in the protective layer as necessary. The protective layer formed from resin can be formed on the outer peripheral surface of the sliding layer, for example, by extrusion coating. On the other hand, a metal such as mild steel is useful as the protective layer. For example, mild steel band may be wound in a spiral form to provide the protective layer. Needless to say, the presence of protective layer has no influence on the frictional force acting on the oil well pipe. EXAMPLE An oil well pipe in accordance with a example of the present invention will be described with reference to FIGS. 1 and 2. FIG. 1 is a sectional view of the oil well pipe of this invention. In this figure, reference numeral 1 denotes an oil well pipe body, 2 denotes a sliding layer, and 3 denotes a protective layer. FIG. 2 is a schematic view illustrating a method for manufacturing the oil well pipe. The oil well pipe is manufactured by applying extrusion coating of sliding layer 2 on the outer peripheral surface of the oil well pipe body 1 by means of a sliding layer extruding machine 4 and by applying extrusion coating of protective layer 3 on the outer peripheral surface of the sliding layer 2 by means of a protective layer extruding machine 5. The steel pipe used is an oil well pipe having an outside diameter of 177.8 mm, a wall thickness of 23.8 mm, and a length of 12,000 mm. On its outer peripheral surface, the sliding layer 2 was formed by extruding melted asphalt, polyethylene, maleic acid modified polyethylene, ethylene-vinyl acetate copolymer, maleic acid modified vinylacetate copolymer, or a mixture of ethylene-vinyl acetate copolymer and maleic acid modified polyethylene by using the sliding layer extruding machine 4. The thickness of the sliding layer is 1 mm for Example Nos. 10, 13, and 15 and Control Nos. 1 and 2, and 2 mm for others. The protective layer 3 was formed on the outer peripheral surface of the sliding layer 2 by extruding melted polyethylene, polypropylene, or ethylene-propylene block polymer by using the protective layer extruding machine 5. The thickness of the protective layer is 4 mm for Example No. 10, and 3 mm for others. Further, an oil well pipe having a protective layer in which a 0.3 mm-thick, 200 mm-wide mild steel band was wound in a spiral form was manufactured. Needless to say, pipes having no protective layer were manufactured as the examples and the controls. Among these examples and controls, Example Nos. 1 to 15 and Control Nos. 1 to 4 are given in Table 1, Example Nos. 1 to 4 in Table 2, and Example Nos. 45 to 60 in Table 3. For the oil well pipes of Example Nos. 1 to 60 and Control Nos. 1 to 4, the stiffness S(t) of the sliding layer at temperatures of 40°C, 50°C, 60°C ,70°C, 80°C, 90°C, 100°C, 110°C, 120 °C, and 130°C after one year has elapsed, and the stability of the protective layer after being left in a furnace of the same temperatures for one hour were examined. The results of these examinations are given in Tables 4 to 6. The oil well pipes of Example Nos. 1 to 60 can be used in the temperature range in which the stiffness S(t) after one year has elapsed is in the range of 10 -5 to 10 3 N/m 2 . In this temperature range, there is little possibility that the oil well pipes are destructed by ground subsidence. It is very safe to use oil well pipes in the temperature range in which the stiffness S(t) is in the range of 10 -5 to 10 2 N/m 2 . Contrarily, it is found that the stiffness S(t) after one year has elapsed exceeds 10 3 N/m 2 at temperatures not higher than 120 °C in Control Nos. 1 and 2 and at any temperatures in Control Nos. 3 and 4, so that the frictional force is high, and the oil well pipes are prone to be destroyed by ground subsidence. The oil well pipes in Example Nos. 1 to 60 show no abnormality in a wide temperature range, so that the damage to the sliding layer and oil well pipe body can be prevented not only when the oil well pipe is transported but also when it is buried. Thus, the oil well pipe having the sliding layer formed from a material consisting mainly of asphalt, polyethylene, maleic acid modified polyethylene, ethylene-vinyl acetate copolymer, maleic acid modified vinylacetate copolymer, or a mixture of these materials is very safe against the destruction caused by ground subsidence when it is used in a temperature range of normal temperature to 130 °C. The present invention prevents the destruction of oil well pipes caused by ground subsidence, and also prevents the damage to the sliding layer and the oil well pipe body not only when the oil well pipe is transported but also when it is buried.

Claims: An oil well pipe for casing comprising an oil well pipe body (1) and a deformable layer (2) formed on the outside of said oil well pipe body (1) wherein, said deformable layer (2) is composed of a material having a viscous behavior at the working temperature of the oil well pipe, said deformable layer (2) is deformed by a shearing stress when a frictional force is applied to the oil well pipe from the outside, said material has a stiffness S(t) after t seconds of 10 -5 to 10 3 N/m 2 after one year or more has elapsed at a working temperature of the oil well pipe, said stiffness S(t) being expressed by the following equation using a deformation amount d(t) mm after t seconds, a frictional force τ N/m 2 and a thickness h mm of the deformable layer: S(t)=3h · τ / d(t) and characterized in that said material is one selected from the group consisting of asphalt, polyethylene having a density of 0.930 g/cm 3 or less, maleic acid modified polyethylene having a density of 0.930 g/cm 3 or less, ethylene-vinyl acetate copolymer having a vinyl acetate content of 5 to 60% by weight, maleic acid modified ethylene-vinyl acetate copolymer having a vinyl acetate content of 5 to 60% by weight and mixtures thereof. The oil well pipe of claim 1, wherein said material is a resin in which ethylene-vinyl acetate copolymer is mixed with maleic acid modified polyethylene. The oil well pipe of claim 1, further comprising a protective layer (3) on the outside of said deformable layer (2). The oil well pipe of claim 3, wherein said protective layer (3) is made from a material having a softening temperature exceeding the working temperature of the oil well pipe. A method for making an oil well pipe for casing comprising the steps of preparing an oil well pipe body (1) and forming a deformable layer (2) on the outside of said oil well pipe body (1) wherein, said deformable layer (2) is composed of a material having a viscous behavior at the working temperature of the oil well pipe, said deformable layer (2) is deformed by a shearing stress when a frictional force is applied to the oil well pipe from the outside, said material has a stiffness S(t) after t seconds of 10 -5 to 10 3 N/m 2 after one year or more has elapsed at a working temperature of the oil well pipe, said stiffness S(t) being expressed by the following equation using a deformation amount d(t) mm after t seconds, a frictional force τ N/m 2 and a thickness h mm of the deformable layer: S(t)=3h · τ / d(t) and characterized in that said material is one selected from the group consisting of asphalt, polyethylene having a density of 0.930 g/cm 3 or less, maleic acid modified polyethylene having a density of 0.930 g/cm 3 or less, ethylene-vinyl acetate copolymer containing vinyl acetate in an amount of 5 to 60% by weight, maleic acid modified ethylene-vinyl acetate copolymer containing vinyl acetate in an amount of 5 to 60% by weight and mixtures thereof. The method of claim 5, wherein said deformable layer (2) is made from a resin in which ethylene-vinyl acetate copolymer is mixed with maleic acid modified polyethylene. The method of claim 5, wherein said deformable layer (2) is formed by an extrusion coating process. The method of claim 5, further comprising a protective layer (3) which is formed on the outside of said deformable layer (2). The method of claim 8, wherein said protective layer (3) is made from a material having a softening temperature exceeding the working temperature of the oil well pipe. The method of claim 8, wherein said protective layer (3) is formed by an extrusion coating process.

Inventors: HORI MITSUAKI, KUROSAWA TOSHIYUKI, NAKAMURA HISAO, OHSUKA SHOICHI, OKANO YOSHIHIRO, HORI, MITSUAKI, KUROSAWA, TOSHIYUKI, NAKAMURA, HISAO, OHSUKA, SHOICHI, OKANO, YOSHIHIRO, HORI, MITSUAKI, C/O INT. PROP. DEPT., NKK CORP., KUROSAWA, TOSHIYUKI, C/O INT. PROP DEPT., NKK CORP, NAKAMURA, HISAO, C/O INT. PROP. DEPT., NKK CORP., OHSUKA, SHOICHI, C/O INT. PROP. DEPT., NKK CORP., OKANO, YOSHIHIRO, C/O INT. PROP. DEPT., NKK CORP.
Applicants: NIPPON KOKAN KK, NKK CORPORATION
Application year: 1994
Main classification: E21B17
IPC codes: F16L9, E21B17